A previously known evaporative fuel processing system recovers evaporative fuel of a fuel tank and introduces the recovered evaporative fuel into an air intake system of an internal combustion engine. The evaporative fuel processing system includes a canister, a purge conduit and a purge valve. The canister recovers the evaporative fuel of the fuel tank. The purge conduit can communicate between an inside of the canister and an inside of the air intake system. The purge valve communicates or discommunicates between the inside of the canister and the inside of the air intake system. For example, JPH11-173220A (corresponding to U.S. Pat. No. 6,138,644A) discloses an evaporative fuel processing system that includes a purge pump, which pumps evaporative fuel recovered by a canister when an air intake system is pressurized by, for example, a supercharger. Furthermore, JP3589632B2 (corresponding to U.S. Pat. No. 6,196,202B1) discloses an evaporative fuel processing system that includes a purge pump, which is driven based on a differential pressure between an inside of an air intake system and an inside of a purge conduit at the time of introducing evaporative fuel recovered by a canister into the air intake system.
However, in the evaporative fuel processing system of JPH11-173220A (corresponding to U.S. Pat. No. 6,138,644A), the purge pump is driven only when the evaporative fuel recovered by the canister is pumped to the inside of the air intake system. Therefore, the purge pump is started from a stop state simultaneously with the time of opening the purge valve. Thus, a delay occurs at the time of introducing the evaporative fuel into the air intake system, and thereby there is a possibility of that the desirable amount of evaporative fuel cannot be introduced into the air intake system.
Furthermore, in the evaporative fuel processing system of JP3589632B2 (corresponding to U.S. Pat. No. 6,196,202B1), the purge pump is driven when the differential pressure between the inside of the air intake system and the inside of the canister is smaller than a predetermined threshold value at the time of opening the purge valve. Therefore, the purge pump is started from a stop state simultaneously with the time of opening the purge valve. Thus, a delay occurs at the time of introducing the evaporative fuel into the air intake system, and thereby there is a possibility of that the desirable amount of evaporative fuel cannot be introduced into the air intake system. Furthermore, in the evaporative fuel processing system of JP3589632B2 (corresponding to U.S. Pat. No. 6,196,202B1), when the differential pressure between the inside of the air intake system and the inside of the canister is larger than the predetermined threshold value at the time of opening the purge valve, the purge pump is not driven. However, when the purge pump is stopped, the stopped purge pump becomes a flow resistance to the evaporative fuel conducted through the purge conduit. Thus, there is a possibility of that the desirable amount of evaporative fuel cannot be introduced into the air intake system.